Primary electrochemical cells are generally well known and available in a variety of sizes and shapes. One well known form of a primary electrochemical cell is a disc or button-type cell. Such a cell typically comprises a physically thin, generally-flat, battery stack disposed within a small, flat, metal housing and exposed to and permeated by an electrolytic solution. Examples of disc or button-type cells are disclosed in U.S. Pat. Nos. 3,907,593; 4,121,020; 4,169,920; and 4,296,187. A typical battery stack as disclosed in each of the above patents includes a flat lithium anode, a flat carbon cathode structure, and a flat separator of an insulative material interposed between the lithium anode and the carbon cathode structure. A preferred electrolytic solution for the above-described cell includes a reducible soluble cathode and an electrolyte solute dissolved in the reducible soluble cathode. Suitable materials for the reducible soluble cathode and the electrolyte solute are thionyl chloride and lithium tetrachloroaluminate, respectively.
While the electrochemical cells of the above-mentioned patents are capable of satisfactory operation, they do not have the capability of providing a warning signal, indication, or other condition toward the end of discharge of the cell indicating that the cell is approaching the end of its useful or rated life. Such an indication may be useful, for example, in applications (e.g., cardiac pacemakers) in which an early and timely replacement of the cell is necessary or desirable.
Cells capable of providing end-of-life warning signals are described above are disclosed in U.S. Pat. Nos. 4,247,607 and 4,293,622. In U.S. Pat. No. 4,247,607, a button-type lithium halide primary cell is disclosed including a flat lithium anode in contact with a flat iodine/polymer depolarizer and having a step portion comprising about 15 to 5% of the total thickness of the anode and a surface area of from about 40 to 60% of the surface area of the major surface in contact with the depolarizer. An indication of approaching end-of-life is provided by a substantial, detectable increase in the internal impedance of the cell when only 5-15% of the unused lithium anode remains. In U.S. Pat. No. 4,293,622, a lithium/thionyl chloride cell is disclosed in which the electrochemically active components of the cell, including the electrolytic solution, are selected so that the electrolytic solution is exhausted during discharge before the other active components. As a result, a detectable step or transition occurs in the output voltage of the cell which serves as an indication of impending cell discharge. A disadvantage of the cell design of this latter patent is that the limiting of the amount of electrolytic solution makes less of the electrolytic solution available for absorbing soluble gaseous discharge products, such as sulfur dioxide, with the result that an undesirable increase in internal cell pressure may occur.